To impress, astound, or entertain someone. To "kick ass." The phrase has its origins in the Bon Jovi song "Wanted Dead or Alive," which contains the unintentionally funny line "I've seen a million faces, and I've rocked 'em all." The phrase was popularized by online humorist Seanbaby, whose website boasts "over 40 million faces rocked!" It seems Seanbaby heard the line in the Bon Jovi song, thought it was funny, and made it his own.
The phrase has since found its way into online dialogue, often used facetiously or ironically (similar to modern use of phrases like "stick it to the Man!"), due to the consciously silly nature of the phrase.
"I've seen a million faces, and I've rocked 'em all." --Bon Jovi
"Over 40 million faces rocked!" -- Seanbaby.com
"This action movie is great. It's gonna rock your face. Hard."